The present invention relates to a solid electrolyte fuel cell. More specifically, it relates to a solid electrolyte fuel cell of flat plate type which operates at high temperatures and can absorb and withstand the thermal expansion of its components at such temperatures. Also, it relates to a solid electrolyte fuel cell in which a thin layer of fine particles is disposed on the side of the oxygen electrode and/or the fuel electrode which side is in contact with the solid electrolyte.
A conventional flat plate type solid electrolyte fuel cell is shown in FIG. 7.
In FIG. 7, numeral 12 indicates a module which is made by piling a plurality of unit cells 1. The unit cell 1 comprises a generating layer 5, a support layer 10 on the fuel side, a support layer 11 on the oxidizer side, and an interconnector layer 9. These elements are piled up and sintered.
This type of conventional solid electrolyte fuel cells have the following problems.
(a) As shown in FIG. 7, the support layer 10 on the fuel side and the support layer 11 on the oxidizer side are arranged perpendicularly with respect to each other so that the fuel 13 (H.sub.2, CO, etc.) and the oxidizer 14 (air, O.sub.2, etc.) do not mix. For the separation of the gases, the generating layer 5 and the interconnector layer 9 are bent at the side portions of the module 12 so that the generating layer 5 is connected to the interconnector layer 9 and the interconnector layer 9 is connected to the generating layer 5 for gas sealing. Since the generating layer 5 and the interconnector layer 9 are only about 100-200 .mu.m in thickness, the gas sealing is not as good as it should be, and the fuel 13 and the oxidizer 14 tend to mix, degrading the cell performance. PA1 (b) The module 12 is formed by piling up unit cells 1. The weight of the module 12 is supported by the support layer 10 on the fuel side and the support layer 11 on the oxidizer side. These support layers are thin ceramics films whose thickness is about 100 .mu.m and have a certain limitation on the piling up of layers because of their fragility. PA1 (c) When an interconnector 101 and a supporting rod 102 and an electrolyte film 103 are simply sintered or cemented together using an adhesive, because of differences in the linear expansion coefficient of the components, each component becomes subjected to large thermal stress. Cracks thus form in the electrolyte film 103, which is to separate a fuel gas 108 and an oxidizer gas 109 completely and which functions as a solid electrolyte. As a result, the two gases mix, and the power generating performance of the cell becomes considerably degraded. PA1 (1) In a fuel cell comprising a generating layer, an interconnector layer, and a support layer disposed between the generating layer and the interconnector, the solid electrolyte fuel cell of the present invention is characterized in that the generating layer is a solid electrolyte sandwiched between a fuel pole and an oxygen pole, the interconnector layer comprises a fuel electrode and an interconnector material and an oxygen electrode, the support layer forms a fuel passage and an oxidizer passage above and below the generating layer, and supporting rods for gas sealing are disposed at both ends of the support layer. PA1 (2) In a solid electrolyte fuel cell comprising a generating layer, an interconnector, and supporting rods disposed between the generating layer and the interconnector, the solid electrolyte fuel cell of the present invention is characterized in that the generating layer comprises a fuel pole, an oxygen pole, and a solid electrolyte film sandwiched between the two poles, the interconnectors and the generating layers form flow passages which cross perpendicularly to each other above and below each generating layer, a sealing film is cemented to the contact surface of the supporting rod between the connection portions of the generating layer and the interconnector, and the sealing film becomes soft or half melted during the operation of the fuel cell. PA1 (3) The solid electrolyte fuel cell as described in (2) above is further characterized in that the sealing film is made and formed of a mixture of inorganic fiber which does not becomes soft even at 1000.degree. C. and inorganic softening powder which becomes soft or half melt at about 1000.degree. C. PA1 (4) Also, the solid electrolyte fuel cell of the present invention comprises a solid electrolyte, a first thin layer which is in direct contact with the solid electrolyte and made up of at least one layer of oxygen electrode fine particles, an oxygen electrode which is placed on the first thin layer and which comprises a layer whose thickness is greater that the first thin layer and which is made of oxygen electrode particles whose diameter is larger than the oxygen electrode fine particles, and a fuel electrode disposed on the main surface of the solid electrolyte opposite from the oxygen electrode. PA1 (5) The solid electrolyte fuel cell of the present invention comprises a solid electrolyte, a second thin layer which is in direct contact with the solid electrolyte and made up of at least one layer of fuel electrode fine particles, a fuel electrode placed on the second thin layer and comprising a layer whose thickness is greater than the second thin layer and which is made of fuel electrode particles whose diameter is larger than the fuel electrode fine particles, and an oxygen electrode disposed on the main surface of the solid electrolyte opposite from the fuel electrode. PA1 (i) The gas sealing surface can be larger as the width of the supporting rod is enlarged. In a conventional cell, the gas sealing surface has been provided only for the width of the generating and interconnector layers. The gas sealing surface whose width is about 200 .mu.m has not been sufficient for good sealing. According to the present invention, the width of the supporting rod is about 5 mm so that the gas sealing surface is much greater and the gas sealing becomes improved. PA1 (ii) Because the weight of the module is supported by the supporting rods which are arranged in parallel crosses in the module, the strength becomes improved and more unit cells can be piled up. The supporting rod of the present invention is stronger compared to a support film because it is an electrolyte body, such as a ceramic body. PA1 (iii) Thermal expansion is no constrained because, while the sealing film is cemented to the supporting rod, the film is merely pressed against the interconnector and the electrolyte with a pressuring load for the sealing surfaces. Thermal stress is therefore very small and does not cause the electrolyte film to break. PA1 (iv) The sealing film shows sufficient sealing effects with a small pressuring load. The sealing film is made of inorganic fiber which does not soften at operating temperature and inorganic softening powder which is a powder of a material which becomes soft at operating temperatures. In the sealing film during operation, the inorganic softening powder, now half melted, fills spaces left empty by the inorganic fiber, and the perfect sealing of gas can be achieved. PA1 (v) In the present invention, as described in (4) and (5) above, fine particles placed on the interface between the electrodes and the electrolyte show the effects we shall describe below.
Furthermore, FIG. 16 shows an example of the conventional solid electrolyte fuel cell which comprises solid electrolyte 201, an oxygen electrode (positive pole) 202, and a fuel electrode (negative pole) 203.
In general, the particle diameter of the particles used in the oxygen and fuel electrodes is not adjusted when conventional fuel cells are manufactured. Therefore, as shown in FIG. 17, the particle diameter is distributed in a broad range which depends on the manufacturing method of the electrode particles. For example, if the mean diameter is 2 .mu.m, the diameter ranges from about 0.2 to about 10 .mu.m. As shown qualitatively in FIG. 18, the spaces between larger particles are filled by smaller particles, and therefore the diffusion of gas through the electrodes is impeded. Also, the effective contact area between the electrode particles and the solid electrolyte is disadvantageously small. As a result, the performance of the solid electrolyte fuel cell tends to be unnecessarily low.